1. Field of the Invention
The present invention relates to a system and method for polishing semiconductor wafers and, more particularly, to the polishing of damascene formed interconnects on the wafer using the chemical-mechanical polishing process in order to achieve a high degree of damascene planarity.
2. Description of Related Art
In the fabrication of integrated circuit devices, numerous integrated circuits are typically constructed simultaneously on a single semiconductor wafer. The wafer is then later divided by cutting the wafer into the individual integrated circuit devices.
Typically, the integrated circuit device is made by a series of layering processes in which metallization, dielectrics, and other materials are applied to the surface of the wafer to form a layered interconnected structure. One important step in the fabrication process is to form interconnects in the insulator layers. The interconnects connect different layers of the integrated circuit device together and provides an integrated circuit device having high complexity and circuit density.
One method for forming the interconnects is to use a damascene method wherein, in general, a via or trench pattern is etched into a planar dielectric layer and then the pattern is filled by metal. Excess metal is typically applied and covers the upper surface of dielectric. The excess metal is then polished away to the patterned metal surface. As with other steps in the fabrication process, it is of extreme importance that the polished interconnect damascene layer be planar.
In order to achieve the degree of planarity required to produce ultra high density integrated circuits, chemical-mechanical planarization processes are now typically employed in the industry. In general, the chemical-mechanical planarization (CMP) process involves pressing a semiconductor wafer against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. Slurries are usually either basic, acidic or neutral and generally contain alumina or silica particles. The polishing surfaces typically are a planar pad made of a relatively soft, porous material such as blown polyurethane. The pad is usually mounted on a planar platen.
In the CMP process, the wafer is typically secured to a carrier plate by vacuum or by a mounting medium such as an adhesive with the wafer having a force load applied thereto through the carrier by a pressure plate so as to press the wafer into frictional contact with the polishing pad mounted on a rotating turntable. The carrier and pressure plate also rotate as a result of either the driving friction from the turntable or rotational drive means directly attached to the pressure plate. In a typical polishing machine, the wafer is transported across the polishing surface to polish the wafer. The CMP process is well known and U.S. Pat. No. 5,423,716 is exemplary and the disclosure of the patent is hereby incorporated by reference.
With regard to the semiconductor devices formed from the wafer, the devices typically include a plurality of interlayered circuits such as metal lines forming an integrated circuit which are interconnected by vias or interconnects between the layers. In the damascene process, metallization of the interconnects is performed by etching the desired circuitry in the dielectric layer down to the active region of the device. A thin layer of conductive metal is deposited by, for example, vacuum evaporation, sputtering or chemical vapor deposition (CVD) techniques, over the entire wafer. The unwanted portions of this metal layer are removed by CMP leaving the thin lines of metals as interconnects.
Both single damascene structures and double damascene layers may be made by the same process and both processes require a CMP process to polish the wafer down to the surface of the interconnects and provide a planar surface. Unfortunately, problems such as dishing occur causing a non-planar surface. The dishing effect is particularly serious since the polishing needs to be carried out until the metal is cleared on the entire wafer, i.e., wherein the metal is left exclusively in previously etched lines without any metal on the surface of the dielectric. It has been found that a significant overpolish is typically needed which results in erosion of dielectric and dishing of metal as much as 100 nm below the level of the dielectric surface. As a consequence, the thickness of the interconnects in overpolished areas is severely reduced resulting in an increased sheet resistance as compared to interconnects in other areas of the wafer and/or the individual integrated circuit device. Additionally, an uneven topography is introduced on the wafer surface after CMP which will be repeated with slight attenuation after subsequent deposition of dielectric layers problems at later steps in the fabrication process.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for polishing semiconductor wafers and other workpieces which have been coated with a layer of metal in a damascene process.
It is an additional object of the invention to provide an apparatus for polishing semiconductor wafers and other workpieces during the damascene step of the fabrication process.
It is another object of the present invention to provide planar workpieces, including semiconductor wafers, made using the improved method and apparatus of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.